Process for analyzing CO2 in seawater

ABSTRACT

The process of this invention comprises providing a membrane for separating CO 2  into a first CO 2  sample phase and a second CO 2  analyte phase. CO 2  is then transported through the membrane thereby separating the CO 2  with the membrane into a first CO 2  sample phase and a second CO 2  analyte liquid phase including an ionized, conductive, dissociated CO 2  species. Next, the concentration of the ionized, conductive, dissociated CO 2  species in the second CO 2  analyte liquid phase is chemically amplified using a water-soluble chemical reagent which reversibly reacts with undissociated CO 2  to produce conductivity changes therein corresponding to fluctuations in the partial pressure of CO 2  in the first CO 2  sample phase. Finally, the chemically amplified, ionized, conductive, dissociated CO 2  species is introduced to a conductivity measuring instrument. Conductivity changes in the chemically amplified, ionized, conductive, dissociated CO 2  species are detected using the conductivity measuring instrument.

This invention was made with U.S. Government support under Grant NumberDE-FG03-93ER81641 awarded by the Department of Energy. The U.S.Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The response of global climate to greenhouse gases, particularly CO₂, isof increasing concern to industrialized nations. Internationalagreements concerning CO₂ emissions will have a major impact ongovernmental policy of advanced nations in the form of environmentalregulation and industrialization policy. Atmospheric and oceanographicscientists are involved in detailed studies of the impact of CO₂ andother greenhouse gases on the global climate. Their findings will shapethe direction the international community takes with regards toworldwide regulation of CO₂ emissions.

Scientific studies of global warming include predictive computer modelsas well as worldwide data collection. Projection of temperature trendssuggest many alternative scenarios dependent on the rate of CO₂increase. The CO₂ increase rate depends on the balance between itsproduction and destruction by a variety of biological and physicalprocesses which are presently poorly understood. Consequently,collection of CO₂ data is critical to evaluating and improving theseclimate models. Global profiles of highly accurate CO₂ data are needed.In particular, water, such as the oceans and other bodies of water,contain much more CO₂ than the atmosphere and may, in fact, act as a CO₂sink or source. The flux of CO₂ across the air-water interface thereforebecomes a very important parameter in determining the rate of change ofCO₂ in the atmosphere.

Current processology relies on exacting sample collection andpreparation, and time consuming analysis. The operation of gaschromatographic or infrared analyzers requires a dedicated analyticallaboratory and expert personnel to attain the necessary level ofaccuracy and precision. These systems are also expensive, difficult toautomate, require frequent calibration, are poorly suited to real timemonitoring of CO₂ in water and/or air, and needs constant maintenance.Most certainly, they are not well suited for wide scale, untendeddeployment in the large numbers necessary to provide data sufficient fora global profile.

Therefore a need exists for a sensor system capable of reliable, highlyaccurate analysis of CO₂ in air and/or water. This can include long termunattended analysis of CO₂ in air and/or water, particularly from aboardship or mounted on buoys or other platforms.

SUMMARY OF THE INVENTION

The above described needs and others are met by the present invention,which is summarized and described in detail below. An air/water CO₂analyzer system has been developed which satisfies the above-describedneeds using a unique process for determining the amount of CO₂. Thisprocess utilizes three basic steps for operation of the subject system,as follows: separating a CO₂ sample by transporting same through amembrane wherein the CO₂ sample is driven by concentration gradientdifferences to form a CO₂ analyte composition, chemical amplification ofthe concentration of ionized by-products of the CO₂ separation, andfinally, specific conductance detection of the amplified CO₂.

The essence of the subject CO₂ analyzer technology lies in the chemicalamplification of the conductivity signal associated with transport ofCO₂ from an air or water source across a membrane into a controlsolution where ionization reactions occur. The control solution containsa chemical reagent which possesses a high absorptivity for CO₂, andwhich as part of the absorption mechanism reacts with CO₂ to formadditional ionic species. For example, alkanolamines comprise a class ofcompounds which are water soluble, which absorb large amounts of CO₂,and which undergo ionization reactions. The chemical amplification ofthe conductivity response of these aqueous solutions will depend onkinetic factors such as contact time, temperature, membranepermeability, etc., the type and concentration of the reagent materials,and the CO₂ concentration. In a given analyzer module and measurementconditions, the conductivity response will be directly proportional tothe CO₂ concentration and the response is detectable using existing flowthrough conductivity measuring devices.

More specifically, a process is provided for determining theconcentration of CO₂ in air or in water. The process comprises providinga membrane for separating CO₂ into a first CO₂ sample phase and a secondCO₂ analyte phase. CO₂ is then transported through the membrane therebyseparating the CO₂ with the membrane into a first CO₂ sample phase and asecond CO₂ analyte liquid phase including an ionized, conductive,dissociated CO₂ species. Next, the concentration of the ionized,conductive, dissociated CO₂ species in the second CO₂ analyte liquidphase is chemically amplified using a water-soluble chemical reagentwhich reversibly reacts with undissociated CO₂ to produce conductivitychanges therein corresponding to fluctuations in the partial pressure ofCO₂ in the first CO₂ sample phase. Finally, the chemically amplified,ionized, conductive, dissociated CO₂ species is introduced to aconductivity measuring instrument. Conductivity changes in thechemically amplified, ionized, conductive, dissociated CO₂ species aredetected using the conductivity measuring instrument. In the preferredprocess of this invention, CO₂ is transported through the membrane froman input side of the membrane to an output side of the membraneemploying a differential concentration gradient in which the dissolvedCO₂ concentration is higher on the input side of the membrane than onthe output side of the membrane causing CO₂ to diffuse across themembrane.

The membrane is typically a microporous or non-porous membrane. Themembrane materials for the non-porous membranes preferably comprisepolytetrafluoroethylene, polydimethylsiloxane, and the membrane materialfor the microporous materials is microporous polypropylene, themicroporous membranes preferably comprising polypropylene hollow fibers.In another form of this invention, the hollow fiber membrane comprises acoaxial tube within a tube arrangement for both gas-liquid andliquid-liquid CO₂ exchange.

The chemical reagent is typically an alkanolamine, preferably analkanolamine selected from a group consisting of primary, secondary, andtertiary alkanolamines with two and three carbon alkanol groups, andmore preferably an alkanolamine selected from a group consisting ofdiethanolamine, monoethanolamine, triethanolamine,N,N-dimethylethanolamine, N-methylethanolamine, N-ethylethanolamine, andDiisopropanolamine.

Other preferred features include a process which is continuous, aprocess in which conductivity changes in the ionized, conductive,dissociated CO₂ species are determined even though unattended forextended periods of time, a process for analyzing CO₂ dissolved inseawater, and a process in which the conductivity measuring instrumentis a flow-through instrument.

Other forms of the invention comprise the following: (1) a closedcircuit configuration in which the chemical reagent solutioncontinuously recirculates through the conductivity measuring instrument,and the chemical reagent solution either gaining or losing CO₂ acrossthe membrane in order to equilibrate with a rising or fallingP_(co).sbsb.2 in the atmosphere or in the water, (2) an open circuitconfiguration in which chemical reagent solution flows from a feedreservoir, first through the membrane, then into the conductivitymeasuring instrument, and then to a waste repository, the chemicalreagent being expendable and making a single pass through the system,and (3) a stopped flow injection configuration in which the chemicalreagent is injected into the membrane contactor, the flow is stopped,and after a predetermined contact time, flow is re-established, a plugof CO₂ -containing chemical reagent solution being displaced from themembrane, and then flowing through the conductivity measuringinstrument. In the stopped-flow process, the contact time is preferablybetween about 5 and 30 minutes.

The preferred system for determining the concentration of CO₂ in air orin water, typically comprises a membrane for separating CO₂ into a firstCO₂ sample phase and a second CO₂ analyte phase; means for transportingCO₂ through the membrane thereby separating the CO₂ with the membraneinto a first CO₂ sample phase and a second CO₂ analyte liquid phaseincluding an ionized, conductive, dissociated CO₂ species; means forchemically amplifying the concentration of the ionized, conductive,dissociated CO₂ species in the second CO₂ analyte liquid phase using awater-soluble chemical reagent which reversibly reacts withundissociated CO₂ to produce conductivity changes therein correspondingto fluctuations in the partial pressure of CO₂ in the first CO₂ samplephase; and a conductivity measuring instrument for receiving thechemically amplified, ionized, conductive, dissociated CO₂ species anddetecting conductivity changes in the chemically amplified, ionized,conductive, dissociated CO₂ species.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow schematic showing a P_(co).sbsb.s Analyzer.

FIG. 2 is a graph showing Inorganic Carbon Speciation vs. pH forDeionized Water at 25 degrees C.

FIG. 3 is a graph showing relationship between P_(co).sbsb.s and K ofdeionized water at 25 degrees C.

FIG. 4 is a flow schematic showing a closed circuit configuration.

FIG. 5 is a flow schematic showing a open circuit configuration.

FIG. 6 is a flow schematic showing a closed circuit test apparatus usedto determine atmospheric P_(co).sbsb.2.

FIG. 7 is a flow schematic showing an open circuit test apparatus usedto determine atmospheric P_(co).sbsb.2.

FIG. 8 is a flow schematic showing an apparatus for equilbration ofsynthetic seawater with atmospheric P_(co).sbsb.2 levels.

FIG. 9 is a flow schematic showing an open circuit test apparatus usedto determine P_(co).sbsb.2 in synthetic seawater.

FIG. 10 is a flow schematic showing an integrated oceanic CO₂ sensortest stand.

FIG. 11A-11C are graphs showing sorption of atmospheric CO₂ by variouschemical reagent solutions.

FIG. 12 is a graph showing three alkanolamines (MEA, DEA, and DIPA) thatwere screened for reversibility characteristics.

FIG. 13 is a graph showing the behavior of a recirculating deionizedwater loop when exposed to varying levels of atmospheric CO₂ rangingbetween 0-10,000 μatm.

FIG. 14 is a graph showing closed circuit atmospheric CO₂ trackingexperiments.

FIGS. 15A-15C are graphs showing raw data, baseline corrected data, andP_(co).sbsb.2 vs time in comparison to IR.

FIG. 16 is a flow schematic showing stopped flow chemical reagentinjection determinations of P_(co).sbsb.2 in sea water.

FIGS. 17A-17C are graphs showing IR derived P_(co).sbsb.2, baselinecompensated detector response, and raw response data respectively.

FIG. 18 is a graph showing a calibration curve constructed at the outsetof the tracking experiment.

FIG. 19 is a graph showing an open circuit atmospheric CO₂ detectionapparatus which was tested with a multiple series of calibration gasesto determine precision, accuracy and the time response characteristics.

FIG. 20 is a graph showing mean specific conductances calculated foreach incremental concentration step, in both ascending and descendingregimes.

FIG. 21 is a graph showing tracking of the changes in ambient laboratoryP_(co).sbsb.2 over the course of approximately 50 hours for a sensorsystem.

FIG. 22 is a graph showing an open circuit dissolved CO₂ detectionapparatus which was tested with a multiple series of calibration gasesto determine precision, accuracy and the time response characteristics.

FIG. 23 is a graph showing a calibration curve and error bars fromreplicate determinations over the full P_(co).sbsb.2 concentration rangebetween 80-750 μatm the standard deviation of the seawater CO₂ analyzer.

FIG. 24 is a graph showing open circuit CO₂ tracking of seawater.

FIG. 25 is a graph showing a cross plot of IR versus CO₂ analyzerP_(co).sbsb.2.

FIG. 26 is a graph showing IR and seawater analyzer P_(co).sbsb.2 valuesover the first eighteen hours of testing.

FIG. 27 is a graph showing IR and seawater analyzer P_(co).sbsb.2 valuesafter the first eighteen hours of the testing.

FIG. 28 is a graph showing specific conductance versus time curves forP_(co).sbsb.2 values between 256-745 μatm.

FIG. 29 is a graph showing FIG. 28 as a three dimensional surface.

FIG. 30 is a graph showing a family of calibration curves plotted foreach contact time.

FIG. 31 is a graph showing seven replicate injections of the threestandard buffers were made at the 30 minute contact time and used toconstruct a calibration curve with error bars.

FIG. 32 is a graph showing synthetic seawater equilibrated with a knownP_(co).sbsb.2 at a contact time of 30 minutes.

FIGS. 33-35 are analyzer time responses to standard P_(co).sbsb.2 stepfunctions summarized in FIGS. 19, and 22 for atmospheric and oceanicopen circuit CO₂ analyzer configurations respectively.

FIG. 36 is a graph showing an open circuit seawater CO₂ apparatus usedto determine variations in instrument response to aqueous levels between80-750 μatm, at 8°, 21°, and 30° C. and presented as a family of curves.

FIG. 37 is the data of FIG. 36 presented as a three dimensional surface.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided an inexpensive,simple, reliable, and accurate means for continuous monitoring of CO₂ inthe atmosphere and water, particularly seawater, with the capability forunattended operation over periods of deployment of up to one year.Operation of the analyzer is based upon previously-described threesequential operations: membrane transport, induced chemicalamplification, and conductivity detection. Concentration gradients driveCO₂ transport from air or water across a membrane into an aqueouschemical reagent solution, typically an alkanolamine solution. CO₂reacts with the dissolved chemical reagent forming charged species whichare then detected using a conductivity cell.

One possible configuration of the CO₂ sensor is illustratedschematically in FIG. 1. CO₂ vapors readily transport across bothnon-porous and microporous hydrophobic membranes. When the membraneseparates a gaseous and an aqueous phase, CO₂ will equilibrate acrossthe membrane as in the following equation: ##STR1##

The equilibrium between free gaseous CO₂ and dissolved CO₂ gas isdescribed by Henry's Law, ##EQU1## where P_(co).sbsb.2 is the partialpressure of CO₂ in the gas phase (atmospheres), m_(co).sbsb.2 is themolality (moles/kg) of CO₂ in the aqueous phase, and H_(co).sbsb.2 isthe Henry's Law constant for CO₂.

If the membrane separates two liquids, and the dissolved CO₂concentration is higher in one liquid than the other, CO₂ will diffuseacross the membrane until equal concentrations are established on bothsides.

In water, ionic species originate from dissolved CO₂ via thedissociation reactions given in the following equations: ##STR2## Theextent of the resulting ionization is strongly pH dependent. Therelationship between pH and inorganic carbon speciation at 25 degrees Cis shown for deionized water in FIG. 2. The dissociation of CO₂ produceshydrogen ions, which in the absence of buffering counter-ions, acidifythe aqueous phase. For example, 300 μatm of CO₂ (g) equilibrating withpure water of an initial pH=7 will produce a final pH of approximately5.8. The ions formed by the dissociation of dissolved CO₂ increase thespecific conductance (K) of the aqueous medium proportional to theirconcentrations.

Following equilibrium transport of dissolved or atmospheric CO₂ across amembrane into deionized water, the conductivity increase resulting fromCO₂ induced ionization reactions will be proportional to the CO₂concentration on the opposite side of the membrane. The relationshipbetween P_(CO).sbsb.2 and K of deionized water at 25 degrees C. isillustrated in FIG. 3 The equilibrium constants K₁, K₂, corresponding tothe two dissociation reactions, the Henry's Law constant H_(CO).sbsb.2,and the equivalent conductances (Λ °) for the ionic species are allfunctions of temperature. Fortunately, both temperature and specificconductance can be measured very precisely using simple and reliableinstruments.

The inherent weakness in using the conductivity of deionized water tomonitor CO₂ stems from the very low levels of specific conductance whicharise due to equilibration with CO₂. This is overcome by chemicalamplification of the basic conductivity signal. Chemical amplificationis achieved using aqueous chemical reagents which increase thesolubility of CO₂ through the ionization reactions shown below. Aqueouschemical reagents such as primary and secondary alkanolamines react withdissolved CO₂ in a two step sequence, forming first a zwitterion, asfollows: ##STR3## which then transfers a proton to an un-ionized amine,forming the corresponding carbamate, as follows: ##STR4## The reactionof tertiary alkanolamines with CO₂ proceeds by the formation of aprotonated amine and a bicarbonate anion, ##STR5##

Conductivity is enhanced by the greater equilibrium concentrations ofthe ionic products for these reactions as compared to those for thedissociation of CO₂ in pure water. Amplification is dependent on thechemical reagent employed and its concentration, the CO₂ concentration,contact time and the kinetic characteristics of the membrane module. Forexample, when 1-4 mM of monoethanolamine solution is used in anexperimental module, the signal was amplified to preferably at leastabout 25 fold, more preferably at least about 35 fold, and mostpreferably at least about 50 fold.

The three step process (membrane transport, chemical amplification, andspecific conductance detection) for determining CO₂ levels in air andwater has been evaluated in the two prime configurations illustratedschematically in FIGS. 4 and 5. These differ only in the flowcharacteristics of the chemical reagent solution. The Closed Circuitconfiguration (FIG. 4) is also represented in FIG. 1. This configurationconsists of a chemical reagent solution which continuously recirculatesthrough the conductivity cell. This represents an equilibrium detectionmode in which the chemical reagent solution gains or loses CO₂ acrossthe membrane in order to equilibrate with a rising or fallingP_(co).sbsb.2 in the surrounding air or water.

In the Open Circuit configuration (FIG. 5) the chemical reagent solutionflows from a feed reservoir, first through the membrane contactor, theninto the conductivity cell, and then to waste. In this configuration theexpendable chemical reagent solution makes a single pass through thesystem. This mode of detection may or may not be an equilibrium process,depending upon the kinetics of CO₂ transport into the chemical reagentsolution in the membrane contactor. Two Open Circuit variants, i.e., theContinuous Open Circuit variant and the Stopped Flow Injection variant,are provided. In the Continuous Open Circuit, the chemical reagentsolution flows at all times.

In the Stopped Flow Injection configuration, the chemical reagentsolution does not flow continuously. A volume of the chemical reagentsolution is injected into the membrane contactor and then flow isstopped. After a predetermined contact time, flow is re-established. The"plug" of CO₂ containing chemical reagent solution is displaced from themembrane contactor, and then flows through the conductivity cell. Thisconfiguration results in a specific conductance peak which isproportional to the P_(co).sbsb.2 of the surrounding air or water.

Experiments were conducted to illustrate the system and process of thepresent invention. As for the reagents and materials employed, a 0.976 %by volume carbon dioxide in oxygen mixture was purchased from AircoSpecialty Gases (Vancouver, Wash.). Two standard mixtures of carbondioxide in air were purchased from Pacific Airgas Inc. (Portland,Oreg.). The higher concentration standard contained 750 parts permillion (ppm) CO₂ by volume in a diluent gas consisting of 19.995 %oxygen (by volume) with the balance (79.930%) consisting of nitrogen.The lower concentration standard contained 200 ppm CO₂ in 20.00% oxygenand 79.998% nitrogen. Lecture bottles of 98.5% nitric oxide, and 99.9%sulfur dioxide were purchased from Aldrich (Milwaukee, Wis.).

Using the van der Waals equation of state, it was determined that errorsdue to non-ideality of CO₂ are not significant at the temperatures andpressures used herein, (i.e. 0.4% for 10,000 μatm at 25 degrees C.).Secondary standard CO₂ gas mixtures were prepared by pressurizing apreviously evacuated gas cylinder using the 0.976% carbon dioxide inoxygen mixture, followed by subsequent pressurizations using firstoxygen and then nitrogen to achieve the desired concentration of CO₂ inair. The oxygen and nitrogen gases used were UN1072 and UN1066 gradesrespectively, purchased from Oregon Airgas Inc. (Roseburg, Ore.). Gasmixtures prepared in this way ranged in P_(co).sbsb.2 between 80-600μatm, with the remainder consisting of approximately 80% nitrogen and20% oxygen. The values obtained by this process were verified bynon-dispersive Infrared (IR) absorption measurements. Care was taken touse only CO₂ -air mixtures owing to the documented potential for errorwhen using nitrogen-CO₂ mixtures in the calibration of IR cells to beused in atmospheric CO₂ determinations.

Two gas mixtures,containing 100 μatm Nitric Oxide (NO), and 100 μatm NOplus 200 μatm CO₂, respectively, were prepared by pressurizing apreviously evacuated lecture bottle using 98.5% NO, followed bysubsequent pressurizations using first oxygen and then nitrogen toachieve the desired concentrations in air. Analogous mixtures containing100 μatm Sulfur Dioxide (SO₂),and 100 μatm SO2 plus 200 μatm CO2 wereprepared. HCl mixtures were prepared from a saturated vapor enclosed ina 1 L. volumetric flask. HCl vapor was removed by gas-tight syringe andused to prepare two gas mixtures within previously evacuated lecturebottles. The two mixtures consisted of 100 μatm of HCl in CO₂ free air,and 100 μatm of HCl plus 200 μatm CO₂ in air, respectively.

Synthetic seawater brine was prepared by dissolving 29.5 g reagent gradeNaCl (VWR Scientific) in 970.5 g deionized water. This solution was notintended to simulate the chemical composition of true seawater, butrather to provide a suitable medium for determining instrument responsesto water samples at high ionic strength.

Determinations of atmospheric P_(co).sbsb.2 were performed using anAstro International Model 5600AT non-dispersive infrared absorptionspectrometer with a gas-tight 13 cm path-length flow through cell. Theinstrument was operated in the 0-1,000 μatm carbon dioxide range. Aconstant sample stream flow rate of 150 cm³ /min was fed to theinstrument by means of either a diaphragm pump or a cylinder ofpressurized gas. Instrumental response time is approximately 3 secondsand repeatability is ±3% full scale or ±30 μatm P_(co).sbsb.2.

The instrument was calibrated using 3 standard gases mixtures. Quadraticcalibration curves were generated using the process of least squares.For on-going calculation of P_(co).sbsb.2, a 40 value look-up table wascreated for the data logger from the calibration curve. This allowed thedata logger to calculate P_(co).sbsb.2 as a function of output voltagewhile experiments were in progress. No corrections were made forfluctuations in temperature or atmospheric pressure.

Total Inorganic Carbon (TIC) was determined using an Astro 2001 System 2total carbon analyzer in the 0-10 mg/L range. This instrument usesacidification and sparging with IR detection. The manufacturer claims arepeatability of 0.2 mg/L for this concentration range.

Bench top specific conductance measurements were made using aCole-Parmer model 19101-00 conductivity bridge and a Cole-Parmer model:G-01481-93 cell. In-line specific conductance measurements on flowingstreams were made using Cole Parmer model MN-01481-66 flow throughconductivity cells and model 19101-00 conductivity bridges. These wereused as integral components of the several test stands described below.The manufacturer claims precision and accuracy for these devices of±0.1, and ±0.2 μS/cm respectively. These instruments providedtemperature compensation for the range between 5°-45° C.

Closed Circuit (Recirculating) CO₂ Vapor Analyzer

The closed circuit test apparatus used to determine atmosphericP_(co).sbsb.2 is illustrated schematically in FIG. 6. The systemconsisted of a manifold of valve selectable calibration gas mixtures andambient air inlet A, a membrane contactor C, a recirculating pump E(ColeParmer model 7520-35), an in-line conductivity cell D and bridge F, anon-dispersive IR cell B, and a data logger G(Molytek model 3702).Separate membrane contactors C, including a vent H to the atmosphere,were prepared from each of the three polymers, PTFE< Siloxane, and uPP.The Siloxane gas-liquid membrane contactor consisted of a single 1524 cmlength of fiber (0.031 cm ID×0.064 cm OD) in a glass shell with aninternal liquid volume of 1.1 cm³, an external gas volume of 1000 cm³,and a membrane gas-liquid contacting surface area of 146 cm². The PTFEgas-liquid membrane contactor consisted of a single 2438 cm fiber (0,051cm ID×0.061 cm OD) in a glass shell with an internal liquid volume of5.0 cm³, an external gas volume of 1000 cm³, and a membrane gas-liquidcontacting surface area of 391 cm². The uPP gas-liquid membranecontactor consisted of a single 610 cm fiber (0.040 cm ID×0.046 cm OD)in a polyethylene shell with an internal liquid volume of 0.8 cm³, anexternal gas volume of 12.1 cm³, and a membrane gas-liquid contactingsurface area of 24.4 cm².

Closed Circuit (Recirculating) CO₂ Detection

Atmospheric detection experiments were conducted using the apparatusdescribed in FIG. 6. Standard compressed CO₂ -air mixtures or ambientlaboratory atmosphere samples were fed to the shell side of thegas-liquid membrane contactor at 150 cm³ /min after flowing through theIR cell. A 0.001M solution of aqueous DEA was recirculated through theinner volume of the hollow fiber membranes and then through an in-lineconductivity cell. Specific conductances, ambient temperatures, IRderived P_(CO2) values, and elapsed times were recorded on disk by meansof the data logger.

Open Circuit (Single Pass) CO₂ Vapor Analyzer

The open circuit test apparatus used to determine atmosphericP_(co).sbsb.2 is illustrated schematically in FIG. 7. The systemconsisted of a manifold of valve selectable calibration gas mixtures andambient air inlet A, a membrane contactor C, a chemical reagent pump E(Cole Parmer model 7520-35), a zero headspace Tedlar chemical reagentfeed reservoir H (Jensen Inert Products, Miami, Fla.), a feedconductivity cell D1 and bridge F, with an outlet to liquid waste I, anin-line conductivity cell D2 and bridge I2, a non-dispersive IR cell B,and a data logger G (Molytek model 3702). In this configuration,specific conductance was determined as the differential between theproperties of influent and effluent chemical reagent. Two gas-liquidcontactor modules were used in this test configuration. A μPP membranecontactor was prepared using a single 90 cm long fiber (0,040 cmID×0.046 cm OD) in a glass shell. This module had an internal liquidvolume of 0.1 cm³, an external gas volume of 1000 cm³ and a membranegas-liquid contact area of 11.3 cm². The second module used was the PTFEmodule described above.

Apparatus for Equilibration of Seawater with CO₂

This apparatus was used as a means of preparation of synthetic seawatersamples which were equilibrated with known atmospheric P_(co).sbsb.2levels. The synthetic seawater brines were then used as influent samplesfor challenge of the Open Circuit Seawater CO₂ Analyzer.

The apparatus, illustrated schematically in FIG. 8, consisted of amanifold of valve selectable calibration gas mixtures and ambient airinlet A, a gas-liquid membrane contactor B with a vent to the atmosphereD, and a brine recirculation pump C (Cole Parmer model 7520-35). Thehollow fiber membrane contactor was constructed using a bundle of 36 μPPfibers in a polycarbonate shell. Each fiber was 22 cm long (0.040 cmID×0,046 cm OD). The contactor internal liquid volume was 1.0 cm³, withan external gas volume of 95 cm³ and a membrane surface area of 100 cm².The high surface to volume ratio in the contactor ensured rapidequilibration compared to other gas-liquid exchange process.

In operation, the shell side of the apparatus was fed either ambientlaboratory atmosphere or compressed gas mixtures at a flow rate of 150cm³ /min. The influent to the apparatus was most commonly the effluentgas from the IR cell. The tube side of the membrane contactor wasinitially charged with synthetic seawater, described above, by means ofa peristaltic pump from a 4 L reservoir. Once charged, the syntheticseawater recirculated through the interior of the contactor forequilibration with the gas phase in accordance with Henry's Law.

Open Circuit (Single Pass) Seawater CO₂ Analyzer

The open circuit test apparatus used to determine P_(co).sbsb.2 insynthetic seawater is illustrated schematically in FIG. 9. The apparatusconsisted of a manifold of valve selectable calibration gas mixtures A,a liquid-liquid membrane contactor F, an chemical reagent pump G (ColeParmer model 7520-35), a zero headspace Tedlar chemical reagent feedreservoir H, a feed conductivity cell I1 and bridge I1, an in-lineconductivity cell J1 and bridge J2, a non-dispersive IR cell B, and adata logger K (Molytek model 3702). In this configuration, specificconductance was determined as the differential between theconductivities of influent and effluent chemical reagents. Additionally,the apparatus consisted of the seawater CO₂ equilibrator previouslydiscussed, including gas/liquid membrane contactor C with vent toatmosphere D, and brine recirculator pump E. The liquid-liquid membranecontactor was fabricated using a single 610 cm length of μPP in afluorinated ethylene propylene (FEP) shell. The internal chemicalreagent volume was 0.8 cm³, with an external synthetic seawater volumeof 11.0 cm³, and a liquid-liquid contact area of 78 cm². In operation,the brine recirculated through both a gas-liquid and a liquid-liquidmembrane contactors, C and F. The first contactor was a device forproducing the desired synthetic seawater P_(co).sbsb.2. Theliquid-liquid contactor transported CO₂ from the brine to the chemicalreagent solution, and was the fundamental step required for thequantitative analysis.

Fully Integrated Test Apparatus

To facilitate the performance of several experiments simultaneously, theCO₂ analyzer configurations illustrated in FIGS. 7 and 9 were integratedinto a single Open Circuit Operational Test Stand (FIG. 10). Theintegrated apparatus incorporated three membrane contactors: thegas-liquid PTFE and μPP modules and the liquid-liquid μPP module. Theintegrated apparatus was used for the determination of both atmosphericand seawater P_(CO).sbsb.2.

Stopped Flow Injection Seawater CO2 Analyzer

The apparatus used for the stopped flow chemical reagent injectiondeterminations of P_(co).sbsb.2 in synthetic. seawater is illustratedschematically in FIG. 11. The system consisted of chemical reagent feedreservoir A, a hollow fiber liquid-liquid membrane contactor E, anchemical reagent feed peristaltic pump B (Cole Parmer model 755360), abrine standard/sample reservoir C, a brine feed peristaltic pump D (ColeParmer), a conductivity cell F, and strip chart recorder G (Omega model585-11-13). The liquid-liquid exchange module was fabricated using a 305cm length of μPP, inside a non-permeable FEP shell. This configurationyielded a shell side (synthetic P_(co).sbsb.2 standard) volume of 57cm³, a tube side internal volume of 0.277 cm³, and a total membranesurface area of 33 cm³.

CO₂ Absorption

An experiment was conducted to compare the relative rates of CO₂ uptakeas a function of concentration for a variety of chemical reagents. 1.0,0.1, and 0.01M solutions of seven alkanolamines diethanolamine (DEA),monoethanolamine (MEA), triethanolamine, N,N-dimethylethanolamine,N-methylethanolamine, N-ethylethanolamine, and Diisopropanolamine wereprepared in deionized water. 100 mL of each solution was thentransferred to a 125 mL unstoppered wide mouth bottle. The alkanolaminesolutions were exposed to the changing CO₂ concentrations of the ambientlaboratory atmosphere for a period of approximately 500 hours. SpecificConductances of the solutions and TIC were periodically monitored andrecorded.

CO2 Absorption Reversibility

To achieve a viable technique for CO₂ determination in the ClosedCircuit (recirculating) configuration, the sorption of CO₂ by thechemical reagent solution must be reversible. To test the relativereversibility trends of CO₂ sorption for several of chemical reagents,in the form of 0,001M alkanolamine solutions, were saturated with CO₂and then sparged with N₂. The change in alkanolamine-CO₂ concentrationwas determined by monitoring specific conductance and TIC. 80 mLaliquots of deionized water and 0,001M aqueous solutions of ethanolamine(MEA), diethanolamine (DEA) and diisopropanolamine (DIPA) weretransferred to 100 mL beakers. Initial conductivity and TIC wasdetermined. The solutions were then sparged with a humidified 1% CO₂ inoxygen gas mixture at a flow rate of 150 cm³ /min, for a period oftwenty minutes. Conductivity and TIC were measured again. The solutionswere then passively exposed to the ambient laboratory atmosphereovernight. The samples were then subjected to a humidified nitrogensparge at a flow rate of 150 cm³ /min for a period of 19 hours. Specificconductance measurements were made after 1, 2, and 19 hours of sparging.

Open Circuit CO₂ Detection In Seawater

Open circuit atmospheric CO₂ detection experiments were conducted usingthe apparatus described above. Standard compressed CO₂ -air mixtures orambient laboratory atmosphere samples were fed to the shell side of thegas-liquid membrane contactor at 150 cm³ /min after flowing through theIR cell. A 0.001M solution of aqueous MEA was pumped from a zeroheadspace Tedlar gas bag feed reservoir, through an in-line conductivitycell, into the hollow fibers within the membrane gas-liquid contactors,and then through a second in-line conductivity cell. In thisconfiguration differential specific conductances were monitored usingthe difference in value between influent and effluent specificconductance measurements. Differential specific conductances, ambienttemperatures, IR derived P_(co).sbsb.2 values, and elapsed times wererecorded on disk by means of the datalogger.

Open Circuit Atmospheric CO₂ Detection

Open circuit detection experiments for monitoring CO₂ in seawater wereconducted using the apparatus described above. Samples of syntheticseawater containing known P_(co).sbsb.2 values were obtained by membraneequilibration using the system delineated above. Standard compressed CO₂-air mixtures or ambient laboratory atmospheric samples were fed to theshell side of the gas-liquid membrane contactor at 150 cm³ /min afterflowing through the IR cell. Synthetic seawater was recirculated throughthe lumen of the gas-liquid membrane contactor at a flow rate of 3 cm³/min. TIC levels in the recirculating synthetic seawater indicatedequilibrium exchange.

The CO₂ equilibrated brine was fed into the shell side of theliquid-liquid membrane contactor. A 0,001M solution of aqueous MEA waspumped from a zero headspace Tedlar gas bag feed reservoir, through anin-line conductivity cell, into the lumen of the hollow fiber membraneliquid-liquid contactor, and then through a second in-line conductivitycell. The effluent from the second conductivity cell was routed to thewaste container. In this configuration differential specificconductances were monitored using the difference in value betweeninfluent and effluent specific conductance measurements. Differentialspecific conductances, ambient temperatures, IR derived P_(co).sbsb.2values, and elapsed times were recorded on disk by means of the datalogger.

Stopped Flow Alkanolamine Injection CO₂ Detection

Synthetic standards with constant P_(co).sbsb.2 were prepared bybuffering both pH and P_(co).sbsb.2 in a 0.01M NaHCO₃ -0.05M boratebuffer solution. The P_(co).sbsb.2 values for these solutions werecalculated from the pH, the first and second dissociation constants forcarbonic acid (K₁ and K₂ defined in equations 1.2 and 1.3), Henry's Lawconstant (H_(CO2)), and the total carbonate species concentration(C_(T)) as in Equation 2.1,

    C.sub.T =[CO.sub.3.sup.= ]+[HCO.sub.3.sup.-]+[CO.sub.2 ]   (Eq. 2.1).

For calculation of P_(co).sbsb.2 it was assumed that the sum of allcarbonate species was equal to the HCO₃ --concentration. This assumptionis valid for the pHs used to buffer this system. The impact on C_(T)(≅0.01M HCO₃ --) of 200 -750 μatm (0.278 to 1.04 mg/L CO₂) isinsignificant. Also, a small volume of 0.001M MEA solution across aliquid-liquid membrane contactor, does not have the capacity to scavengesufficient CO₂ from this solution to change its concentration. The 0.05Mborate buffer is also little affected by changes in the dissolved CO₂,and as a result, the equivalent inorganic carbon speciation andP_(co).sbsb.2 in the buffered brine solution is described by the systemidentified in Equations 2.2-2.5. ##EQU2##

The pHs of three different 0.01M NaHC₃ -0.05M borate buffer solutionswere adjusted to give P_(co).sbsb.2 values of 256, 544, and 745 μatm.Although these CO₂ values are not compensated for solution non-idealityand consequently are accurate to only two significant figures, they areconstant for each buffer solution and their use in generation ofstatistical deviations is valid.

The stopped flow chemical reagent injection liquid phase CO2 detectionexperiments were conducted using the apparatus described above. Theshell side of the liquid-liquid membrane contactor was filled withP_(co).sbsb.2 buffer solutions. An aqueous 0,001M MEA solution waspumped into the lumen of the liquid-liquid membrane contactor at a flowrate of 0.56 cm³ /min. The flow was then stopped for a specified contactperiod. These experiments were conducted using contact times of 5, 10,15, 20, 30, and 50 minutes. At the end of the contact time period, thechemical reagent solution was pumped through the in-line conductivitycell, and the corresponding conductivity peak recorded. Characterizationof Temperature Effects.

A liquid-liquid membrane contactor was constructed consisting of asingle μPP 305 cm fiber inside an impermeable FEP shell of equal length.This configuration yielded an internal liquid volume of 0.4 cm³ and anexternal shell side liquid volume of 5.6 cm³, with a 32 cm² membranesurface area. The membrane contactor was integrated into an open circuitapparatus similar to that described above, but housed inside acommercially available 48 quart ice chest, filled with 8±1° C. water,fitted with thermal equilibration coils, and instrumented with a K- typethermocouple.

Synthetic seawater was equilibrated with gases of varying CO₂ contentusing the apparatus previously described. The sample was then circulatedthrough first a 150 cm thermal equilibration coil, and then the shellside of the membrane contactor located within the cooler. A 0.001M MEAsolution was pumped from the gas tight reservoir at room temperature,through the first in-line conductivity cell, then through the 150 cmthermal equilibration coil, through the lumen of the hollow fibermembrane contactor, and into a temperature compensated conductivity cellco-located within the cooler, just above the water line.

Elevated temperature measurements were obtained in a similar mannerusing the apparatus with an internal water bath temperature of 30±1° C.Room temperature measurements were obtained at 21±1° C.

Interferences

The μPP membrane contactor and open circuit gas-liquid CO₂ detectionapparatus described above were used to determine the interferenceeffects of HCl, NO, and SO₂ on the specific conductance of chemicalreagent solutions. The apparatus was operated in the normal CO₂detection mode. For these experiments the apparatus was installed insidea fume hood. Similar experiments were performed using each of the acidgases.

The apparatus was first exposed to 200 μatm CO₂ in air until a steadyoutput conductivity signal was achieved. Next, the apparatus waschallenged with a gas mixture containing 100 μatm of the subject acidgas plus 200 CO₂ in air. This was followed by exposure to anitrogen-oxygen mixture, free of both the subject acid gas and CO₂,until a stable conductivity signal was again achieved. Lastly a mixturecontaining 100 μatm of the subject acid gas in CO₂ free air was sampled.The differential specific conductances under each of these conditionswere recorded.

Solid Phase Calibration Experiments

A liquid-liquid contactor was fabricated consisting of one 762 cm μPPtube inside an FEP shell of equal length. Internal volume and surfacearea for the hollow fiber were 0.7 cm³ and 96 cm² respectively. Externalshell side volume was 55 cm³. A packed bed containing 5 cm³ of acrystalline calcium carbonate based solid phase basification material(manufactured by Umpqua Research Company) was prepared inside a 0.64 cmOD FEP tube. A similar bed was prepared from a crystalline molybdenumtrioxide based solid phase acidification material (manufactured byUmpqua Research Company). These two components were plumbed in series.Using a peristaltic pump, degassed deionized water was pumped, from agas tight reservoir, through the calcium carbonate bed for thecontrolled dissolution of inorganic carbon, and then through themolybdenum trioxide bed for acidification of the stream. The effluentfrom the acidification module was then routed through the lumen of thehollow fiber liquid-liquid contactor. 0.001M MEA was pumped by aperistaltic pump from a gas tight reservoir through an influentconductivity measuring cell, into the external shell of the membranecontactor, through a conductivity cell, and finally to an effluentcollection reservoir. Differential specific conductance, pH and TIC weremeasured.

EXAMPLE 1

The sorption of atmospheric CO₂ by chemical reagent solutions wasdetermined. This was done by measuring the CO₂ absorptive properties ofseven alkanolamines. The alkanolamines included primary, secondary, andtertiary alkanolamines bearing two and three carbon alkanol groups, andethanolamines bearing methyl, dimethyl, and ethyl alkyl groups. Resultsof the CO₂ absorption tests, using these substances at concentrationsspanning a three decade range between 1.0 and 0.01M, are givengraphically in FIGS. 11A-11C. Similar results were shown for thesealkanolamines with respect to concentration. Higher molarities ofalkanolamine result in higher specific conductances at CO₂ saturation.Ethanolamine, N,N-dimethyl ethanolamine, and diethanolamine are the mostpreferred species for translating CO₂ sorption into a specificconductance signal.

For the purposes of incorporation into an analytical instrument,favorable time response characteristics are as important as strength ofthe primary measurement signal. These CO₂ sorption experiments highlightthe benefit of lower chemical reagent concentrations for an applicationin which the fastest achievable response characteristics are desired.

EXAMPLE 2

For operation of the CO₂ sensor by equilibration of an chemical reagentsolution with a gaseous or dissolved CO₂ sample, the absorption must bereversible, and the kinetics of desorption must be sufficiently rapid toachieve an acceptable detector time response. Three alkanolamines (MEA,DEA, and DIPA) were screened for reversibility characteristics. Theexperimental results are presented graphically in FIG. 12. Testing wasconducted using the tube within a tube configuration, consisting of asingle hollow fiber membrane strand located inside a length of largerimpermeable tubing. Hollow fiber bundles were used for saturation ofsynthetic seawater with CO₂. The first sensor configuration examined wasthe recirculating chemical reagent closed circuit.

Prior to experimentation with chemical reagents, the behavior of arecirculating deionized water loop was characterized when exposed tovarying levels of atmospheric CO₂ ranging between 0-10,000 μatm. Amicroporous polypropylene membrane was used. As shown in FIG. 13, theexperimental results correspond to theoretical values predicted byHenry's Law, the CO₂ dissociation equilibria, and equivalentconductances at infinite dilution for H+, OH--, HCO₃ --, and CO₃ ═. Thisconfirmed the operability of the experimental apparatus.

EXAMPLE 3

Closed circuit atmospheric CO₂ tracking experiments were conducted usingthe apparatus illustrated in FIG. 6, 0.001M DEA solutions, andgas-liquid contactors utilizing non-porous polydimethylsiloxane,non-porous polytetrafluoroethylene, and microporous polypropylenemembranes. Each CO₂ detection circuit was exposed to the ambientlaboratory atmosphere and the change in conductivity versus time wasmonitored. In all cases a continuously increasing conductivity baselinewas observed due to transport of water vapor across the semipermeablemembranes into the gaseous phase. This resulted in a net concentrationof the DEA solution with a corresponding increase in baseline specificconductances. As illustrated in FIG. 14, the relative susceptibilitiesof the three membranes to water loss follows the relationSiloxane >>μPP >PTFE. The rates of baseline elevation for both theSiloxane and μPP membranes were sufficiently large that no useful CO₂tracking data was obtained.

The rates of water loss across the PTFE membrane were sufficiently slowas to allow a reasonably accurate quantitative tracking of changing CO₂concentrations. This was achieved by baseline compensation using alinear equation, and calibration of the closed circuit CO₂ sensor bycomparison of IR derived CO₂ values and sensor derived specificconductances. The raw data, baseline corrected data, and P_(co).sbsb.2vs time in comparison to IR are shown in FIG. 15A-15C, respectively.

In this tracking experiment, the Closed Circuit CO₂ Vapor Sensor wasoperated continuously for a period of approximately 60 hours. ChangingP_(co).sbsb.2 levels were tracked over this time period using the IRcell. The conductivity versus time traces of FIG. 15A-15B, and theP_(co).sbsb.2 versus time trace of FIG. 15C, indicate cyclicfluctuations in atmospheric P_(co).sbsb.2, with a 24 hour periodicity.Each of these maxima correspond to levels of peak daytime activitywithin the laboratory and reflect maximum P_(co).sbsb.2 values in therange between 480-490 μatm. Periods of minimum P_(co).sbsb.2 correspondto evenings and nights. At these times P_(co).sbsb.2 within thelaboratory fell to values between 380-400 μatm.

Inspection of FIGS. 15A-15C clearly indicate that the Closed Circuit CO₂Vapor Sensor faithfully tracked the relative fluctuations of thelaboratory atmospheric P_(co).sbsb.2 over the full time course of thisexperiment. As shown in the cross-plot of IR derived P_(co).sbsb.2versus Vapor Sensor P_(co).sbsb.2 given in FIG. 16, the two agreed towithin ±5%. This level of accuracy was obtained under circumstances inwhich the deviation in specific conductance between P_(co).sbsb.2 maximaand minima was less than 1.5 μS/cm. Given the non-optimizedconfiguration of the sensor used in this experiment, the results aretaken as strong indication of the potential of this process for preciselong term quantitative determination of atmospheric P_(co).sbsb.2.

EXAMPLE 4

Open circuit atmospheric CO₂ detection was conducted, as described inExample 4, using a μPP membrane contactor and an open circuitatmospheric CO₂ detection configuration, similar to that illustrated inFIG. 7, but with only a single in-line conductivity cell. In thisconfiguration, 0.001M MEA was used for chemical amplification. In theopen circuit configuration the chemical reagent solution does notrecirculate. Instead, it flows through the membrane contactor into theconductivity cell, and then to waste. For this reason, reversibility ofthe CO₂ absorption is not important, and hence the more active but lessreadily reversible MEA was used.

The open circuit atmospheric CO₂ tracking results are shown graphicallyin FIG. 17. FIGS. 17A-17C present IR derived P_(co).sbsb.2, baselinecompensated detector response, and raw response data respectively.Inspection of FIG. 17C indicates a curvilinear rising baseline. This wasdue to absorption of CO₂ vapor into the chemical reagent feed solutionprior to flow through the membrane contactor. These results prompted themodification of the open circuit atmospheric CO₂ detection circuit tothe configuration illustrated in FIG. 7. Two changes were incorporatedinto the new open circuit configuration. These were 1) storage of MEAfeed solution in a gas-tight, zero headspace Tedlar bag, and 2)installation of a second in-line conductivity cell at the outlet of theMEA feed reservoir. All subsequent specific conductance measurementswere taken as the differential between the two conductivity detectors.

The relative complexity of the curvilinear baseline elevation for thistracking experiment precluded the calculation of P_(co).sbsb.2 from thespecific conductance data as had been reported in the previous sectionfor the closed circuit experiment using the PTFE membrane contactor. Anattempt at baseline compensation was made using a quadratic equation.The resulting partially baseline compensated conductivity trace is givenin FIG. 17B. While calculation of P_(co).sbsb.2 from these data couldnot be accomplished with any degree of accuracy, the complimentarysymmetry between the IR output and chemical reagent conductivity tracesis taken as a strong indication of the potential for long termatmospheric CO₂ monitoring using the latter process. A calibration curveconstructed at the outset of the tracking experiment is illustrated bythe closed circles in FIG. 18.

The change in test apparatus design leading to the configurationillustrated in FIG. 7, resulted in drastically improved performance inthe open circuit CO₂ detection equipment. The second in-lineconductivity detector provided continuous monitoring of the baselinespecific conductance of the MEA solution flowing into the membranecontactor. This was intended as a means of compensating for baselineelevation due to the absorption of atmospheric CO₂ into the MEA solutionupstream of the membrane contactor. This was found to be completelyunnecessary as the specific conductance at the outflow face of the MEAfeed reservoir remained constant for periods of up to six weeks. Theaddition of the Tedlar gas-tight zero headspace bag as the MEA feedreservoir effectively prevented any undesirable contact between the MEAfeed solution and atmospheric CO₂.

EXAMPLE 5

The improved open circuit atmospheric CO₂ detection apparatus was testedwith a multiple series of calibration gases in order to determineprecision, accuracy and the time response characteristics of the system.These tests took the form of step functions as illustrated in FIG. 19for the μPP membrane contactor. Calibration air-CO₂ mixtures containing80, 200, 510, and 750 were sequentially fed to the apparatus for timesof approximately 45 minutes. During this time, differential specificconductance and IR derived P_(co).sbsb.2 were continually monitored.Concentrations were increased stepwise from the 80 minimum to the 750μatm maximum, and then decreased stepwise to the minimum.

Mean specific conductances were calculated for each incrementalconcentration step, in both ascending and descending regimes. Thisresulted in the collection of multiple data points for eachconcentration. From these data a calibration curve was constructed witherror bars (FIG. 20). As expected, relative standard deviations(coefficients of variation) decreased with increasing sampleconcentrations. The values ranged from 6.4% at 80 μatm to 4.0% at 750μatm. Significantly, it is evident from an inspection of FIG. 19 that,with the exception of the 750 μatm concentration, once the specificconductance stabilized at the new level, the experimental apparatus gavemore precise results than the IR. These experiments reflect anapproximate 30-fold chemical amplification of specific conductance overdistilled water values. No attempt was made to compensate forfluctuations in barometric pressure, or ambient temperature.

Following completion of the step function standard air-CO₂ gas tests,the sensor system was again allowed to track the changes in ambientlaboratory P_(co).sbsb.2 over the course of approximately 50 hours. Theresults are indicated in FIG. 21. The similarity of symmetry between theIR and experimental analyzer output is a strong indication of thepotential for development of a highly accurate P_(co).sbsb.2 sensorsystem.

EXAMPLE 6

This example illustrates the Open Circuit Seawater CO₂ Detection system.The open circuit apparatus and μPP membrane contactor used for thedetection and quantitation of CO₂ in synthetic seawater is illustratedin FIG. 9. Preliminary shakedown of the apparatus was conducted usingdeionized water equilibrated with atmospheric CO₂ as the aqueous sample.This work generated the calibration curve shown in FIG. 18, indicatinggood agreement between both the atmospheric and seawater detectionsystems, when operated in the open circuit configuration. All subsequentaqueous phase CO₂ determinations were conducted using synthetic seawater

Similar experiments using the standard air-CO₂ gas mixtures ranging inconcentration between 80-750 μatm were conducted for the liquid phasedetection system. The results are shown in FIG. 22. In these experimentsit is noteworthy that the apparent slow response of the overall systemis in reality an artifact of the means used to produce the liquid samplecontaining known P_(co).sbsb.2 values. In this configuration, therecirculating seawater sample must first equilibrate with the standardgas before the liquid-liquid CO₂ exchange membrane can be expected tofully respond. Thus, the overall system response is the convolution oftwo dependent membrane response functions, corresponding to samplecreation and analysis respectively. The overall system response time isapproximately double that expected for stabilization of thetransmembrane CO₂ transport from the synthetic brine to the MEAsolution.

Inspection of FIG. 22 shows a very good correspondence between the knownP_(co).sbsb.2 of the calibration gases and the seawater CO₂ analyzeroutput. This is also reflected in the calibration curve and error barsfrom replicate determinations shown in FIG. 23. Over the fullP_(co).sbsb.2 concentration range between 80-750 μatm the standarddeviation of the seawater CO₂ analyzer varied between 4.35-5.57 μatm,corresponding to coefficient of variation maxima and minima of 7.0% and0.7% respectively. These varied between 2.5% and 0.9% in the 200-500μatm concentration range in which most real seawater values are expectedto fall. Given the lack of optimization of the apparatus andprocessology, and the use of components such as pumps which were farfrom ideal in their performance, these results provide a very positiveindication of the high probability that further development will resultin a seawater CO₂ analyzer capable of providing data of the qualityneeded.

For a period of approximately 35 hours the open circuit seawater CO₂analysis system was allowed to track the fluctuations in syntheticseawater P_(co).sbsb.2 induced by the diurnal fluctuations in theambient laboratory atmospheric CO₂ concentration. In this test, thesynthetic seawater was first equilibrated with changing atmospheric CO₂.P_(co).sbsb.2 in the brine was then determined by the instrument. Thetracking results are given as a time series in FIG. 24 and as a crossplot of IR versus CO₂ analyzer P_(co).sbsb.2 in FIG. 25. Very goodagreement was obtained between IR and seawater analyzer P_(co).sbsb.2values over the first eighteen hours of the test (FIG. 26). Aftereighteen hours, the values diverged, maintaining similar symmetry, butwith an offset until the conclusion of the tracking experiment (FIG.27). The cause of the offset occurring midway through the test was foundto be a change in flow rate of the chemical reagent pump ofapproximately 0.005 cm³ /min. Future development of this technology mustincorporate more reliable fluid delivery such as from syringe pumps,high performance liquid chromatography (HPLC), or osmotic pumps.

EXAMPLE 7

Stopped flow chemical reagent injection experiments were conducted todetermine the role of kinetics in single pass systems, and also toevaluate a third possible configuration for both atmospheric and oceanicCO₂ quantitation. Using this processology a volume of chemical reagentis injected into the membrane contactor for a preset time period, afterwhich the CO₂ loaded chemical reagent is pumped through an in-lineconductivity cell. Detector output is in the form of a nearly gaussianpeak, resulting from the "plug" of partially saturated chemical reagentsolution displaced from the membrane contactor.

The kinetics of 0.001M MEA -CO₂ membrane transport and re;action wasexamined using aqueous P_(co).sbsb.2 buffer solutions: and the apparatusshown in FIG. 11. The resulting specific conductance versus time curvesare shown in FIG. 28 for P_(co).sbsb.2 values between 256-745 μatm., andin FIG. 29 as a three dimensional surface. The most obvious feature ofthis family of curves is the concentration dependence of the responserate. For high CO₂ concentration gradients, shorter response times wereneeded to approach equilibrium specific conductance values when comparedto lower CO₂ concentration differentials.

These data were used to prepare the family of calibration curves shownin FIG. 30, plotted for each contact time. At a five minute contacttime, the response is nearly linear. Increasing the contact time to 10and 15 minutes increases the relative response, although at the highestCO₂ concentration, the curve begins to flatten. This behavior becomeseven more pronounced at the longer contact times. The curvature is dueto the increasing conducting species formed by the reaction of MEA withCO₂ as well as the degree of saturation for MEA absorption of CO₂. Theflux of CO₂ across the membrane is only sufficient at the highest CO₂concentration gradients and the longest contact times to saturate theMEA. As the concentration of ionic species increases, activitycoefficients and hence equivalent conductances (Λ_(i)) of the protonatedamine, carbamate, bicarbonate, and carbonate decrease due to ion-ioninteraction. This results in diminished differential specificconductance response at higher P_(co).sbsb.2.

When the chemical reagent injection contact times were extended to muchlonger times (i.e. 60-300 minutes), the 745 and 256 μatm curvesapproached a constant differential conductance. In this situation theindividual specific conductances continued to increase with time. Thisbehavior is due to the flux of water across the membrane. Thedifferences in osmotic pressure between the MEA and buffer solutionsdrive the transfer of water from the MEA solution to the P_(co).sbsb.2buffer. This results in an increase in MEA concentration.

EXAMPLE 8

Seven replicate injections of the three standard buffers were made atthe 30 minute contact time. These data were used to construct thecalibration curve with error bars shown in FIG. 31. The standarddeviations for the 256, 544, and 745 μatm buffers are 1.49, 9.91, and21.85 μatm respectively. The standard deviation as a function ofP_(co).sbsb.2 is an increasing exponential relation of the form,

    σ==4.76+2.93 exp(0.00296 P.sub.co.sbsb.2),

with a correlation coefficient (r²) of 1.0000. Using this expression toevaluate σ at a partial pressure of 350 μatm yields σ=3.5 μatm.

The chemical reagent injection procedure was also used with syntheticseawater samples equilibrated with a known P_(co).sbsb.2 at a contacttime of 30 minutes. The peak heights for replicate injections at valuesof 200 and 510 μatm P_(co).sbsb.2 are shown in FIG. 32. This processclearly shows promise as a means of obtaining precise quantitation ofoceanic P_(co).sbsb.2.

EXAMPLE 9

The analyzer time responses to standard P_(co).sbsb.2 step functionssummarized in FIGS. 19, and 22 for atmospheric and oceanic open circuitCO₂ analyzer configurations respectively and presented in greater detailin FIGS. 33-35 for the latter configuration, indicate equivalentresponse characteristics irrespective of whether the P_(co).sbsb.2values are increasing or decreasing. The response times also were foundto be relatively independent of the concentration differences betweeninitial and final conditions. The response times shown for the seawaterCO₂ analyzer are prolonged by an undetermined factor, owing to theprocess used for equilibrating the synthetic seawater with atmosphericP_(co).sbsb.2.

EXAMPLE 10

An open circuit seawater CO₂ apparatus was used to determine variationsin instrument response to aqueous levels between 80-750 μatm, at 8, 21,and 30° C. The experimental results are presented as a family of curvesin FIG. 36, and as a three dimensional surface in FIG. 37. Notsurprisingly, the specific conductance associated with any givenP_(co).sbsb.2 value rises with increasing temperature. The measureddifferences in specific conductance are significant over relativelysmall temperature changes. From these data it is very clear that precisetemperature measurement, and means of temperature compensation must beincorporated into the device.

EXAMPLE 11

Three acid gases were investigated for interference with the subjectprocessology: HCl, SO₂, and NO. The experimental results are summarizedin Table I. Two of these substances, SO₂ and NO, were found to bestrongly interfering. These gases readily transported across themembrane, and formed ionic species by the following reactions:

    SO.sub.2 +H.sub.2 O→H.sup.+ +HSO.sub.3 --

    3 NO→N.sub.2 O+NO.sub.2

    NO.sub.2 +NO+H.sub.2 O→2H.sup.+ +2NO.sub.2 --

The interference noted is due to the formation of completely dissociatedstrong mineral acids. Because these species form ionic and hencenon-membrane transportable species in aqueous solution, they should notexhibit a significant interference when aqueous phase samples areanalyzed.

EXAMPLE 12

A packed bed of calcite crystals (CaCO₃ ) was evaluated as a potentialsource of in-line calibration for the chemical reagent based CO₂analyzer. Such a bed will impart a controllable amount of CO₃ ═ to theeffluent. A molybdenum trioxide based solid phase acidifier, alsoincorporating proprietary technology, was located immediately downstreamto shift the inorganic carbon equilibria from CO₃ ═ and HCO₃ -- towardCO₂. In a gas free and gas-tight environment, DI water was pumpedthrough the solid state modules and into an open circuit dissolved CO₂detection apparatus flowing 0.001M MEA solution. The experimentalresults are summarized in Table II. The calibration system produced anaqueous inorganic carbon concentration of 1.4 mg/L (as C), and resultedin a differential specific conductance signal of 22.5 μS/cm. Inspectionof the calibration curve shown in FIG. 23 indicates that this valuecorresponds to a P_(co).sbsb.2 of approximately 450 μatm. This2preliminary result is encouraging. With the use of carbonate species ofdiffering solubilities, the subject solid phase modules can be made toproduce the desired range of aqueous P_(co).sbsb.2.

Regarding Examples 1-13, the quantitative determination of CO₂ inatmospheric and aqueous samples using membrane transport, chemicalreagent induced chemical amplification, and conductivity detection hasbeen demonstrated. Three chemical reagent flow configurations have beenevaluated: closed circuit with continuous chemical reagentrecirculation, open circuit with continuous chemical reagent flow in asingle pass through the memkrane contactor, and stopped flow chemicalreagent injection.

Three membrane materials were evaluated. These included nonporouspolytetrafluoroethylene (PTFE), polydimethylsiloxane (Siloxane), andmicroporous polypropylene (μPP). Microporous polypropylene hollow fibersproved to be the most preferred, primarily due to high CO₂ transportrates and large surface area to volume ratio. The most usefulconfiguration for the hollow fiber membrane contactor was found to bethe coaxial tube within a tube arrangement for both gas-liquid andliquid-liquid CO₂ exchange.

Primary, secondary, and tertiary alkanolamines with two and three carbonalkanol groups were evaluated for conductivity response, reversibilityof CO₂ absorption, and concentration effects. Monoethanolamine (MEA) wasidentified as the most preferred alkanolamine based on specificconductance changes corresponding to a fixed P_(co).sbsb.2.Diethanolamine (DEA) and diisopropanolamine (DIPA) were found to be themost readily reversible of the alkanolamines tested. DEA was selectedfor use in the closed loop detector configuration. MEA was used in theopen circuit and stopped flow chemical reagent injection configurations,since reversibility is not required.

The chemical reagent concentration determines the total CO₂ absorptioncapacity of the solution. The specific conductance of the chemicalreagent solution is directly but non-linearly proportional to thequantity of CO₂ absorbed. The deviation from linearity in the detectorcalibration curves are due to the combined effects of membrane transportrates, ionization reaction kinetics, the degree of saturation of thechemical reagent solution, decreased activity coefficients at higherconcentrations, and the multiple equilibria associated with CO₂dissolution and the formation of ionic species. Contact times andchemical reagent concentrations are the two prime variables which can bemanipulated to shift the desired dynamic P_(co).sbsb.2 range of theanalyzer into the steep and nearly linear portion of the calibrationcurve. This allows optimal precision and accuracy to be attained.

For a given continuous flow open loop membrane contactor, contact timevaries with the chemical reagent flow rate. Chemical reagent flow ratesof approximately 0.01 mL/min were used. The peristaltic pumps availablefor the feasibility demonstration were not capable of providing constantflows at these low rates. Very significant improvements in precision andaccuracy of the CO₂ analyzer can be expected from the use of moresophisticated low flow rate pumping systems such as syringe pumps, HPLCpumps, or osmotic pumps.

Contact time in the stopped flow chemical reagent injection analyzerconfiguration is controlled by clock and can be expected to beaccurately controlled by a microcontroller or microcomputer. Using0.001M MEA, valid calibrations were obtained over a range of contacttimes between 5 and 30 minutes. For a given membrane contactor design,contact time and MEA concentration can be optimized to provide thehighest level of accuracy for the range of interest. Short contact timesrequire more accurate timing of events than do longer times.

Temperature influences diffusion, dissolution, and reaction rates.Temperature also affects CO₂ solubility, ionization reaction equilibria,and equivalent conductances of all ionic species. Improved detectorperformance can be obtained by incorporation of a highly accuratetemperature measurement device, such as an RTD, into the CO₂ analyzerdesign. A temperature compensation algorithm is required to correctdetector response for the changes in analyzer performance associatedwith temperature fluctuations of the operational environment.

Interferences from acid gases such as NO and SO₂ were evident for theatmospheric CO₂ detector. These interferences are due to the formationof completely dissociated strong mineral acids which are highlyconductive. Because these gases form ionic species in aqueous solutions,they are not amenable to vapor phase membrane transport, and hence arenot expected to interfere with the determination of CO₂ in seawater.

Remote calibration capability for a buoy mounted analyzer greatlyimproves system accuracy over prolonged periods of deployment. Thecontrolled dissolution of crystalline CaCO₃ packed into a flow-throughmodule, acidification of the stream using a similar bed of crystallineMoO₃, and membrane transport of the resulting CO₂ has been shown toproduce a specific conductance response equivalent to a P_(co).sbsb.2 ofapproximately 450 μatm.

The open circuit analyzer configuration yielded a precision of ±4-5 μatm(1σ) between 80 and 750 μatm P_(co).sbsb.2 for synthetic seawatersamples, and ±5-30 μatm for this range of P_(co).sbsb.2 in air. The opencircuit analyzer is capable of tracking CO₂ fluctuations in air andwater for extended periods of continuous operation. Important factorsfor establishing open circuit analyzer configuration include: precisecontrol of flow rates, compensation for temperature and pressureeffects, and optimization of the chemical reagent concentration sc thatP_(co).sbsb.2 lies within the steep and nearly linear portion of theP_(co).sbsb.2 versus specific conductance curve (FIG. 18). Responsetimes of approximately 15 minutes were obtained for step functionchanges in P_(co).sbsb.2. This value can be improved upon throughoptimization of the mass transport characteristics of the membranecontactor. Significantly, P_(co).sbsb.2 values provided by the opencircuit CO₂ analyzer followed the known concentrations of standard CO₂-air mixtures during step changes more accurately than did ournon-dispersive IR detector. The sensitivity of the open circuit CO₂analyzer can be attributed to the substantial chemical amplification ofthe conductivity signal. The specific conductance of deionized waterequilibrated with an atmospheric P_(co).sbsb.2 of 350 μatm isapproximately 0.9 μS/cm, while a 0.001M MEA solution in the open circuitconfiguration exposed to 350 μatm has a differential specificconductance of approximately 32 μS/cm, corresponding to a 35-foldamplification.

In this case, the standard deviation increased exponentially withconcentration. Based upon this relationship, approximate error valuesfor 350 μatm P_(co).sbsb.2 of ±3.5 μatm were obtained. Performance ofthe chemical reagent injection CO₂ analyzer configuration can beimproved significantly through temperature and pressure compensation,and optimization of the chemical reagent molarity and contact time toshift the desired CO₂ concentration range into the linear portion of thecalibration curve (FIG. 31). Response times of the stopped flow analyzerwere between 5 and 30 minutes.

An attractive feature of the open circuit configuration is operationalsimplicity and the consequent ease of operational control. The stoppedflow configuration offers potential gains in decreased chemical reagentconsumption rates, more precise contact time control, and relaxed flowcontrol requirements. Both configurations are compatible with long termdeployment for prolonged periods of unattended operation. The chemicalreagent flow rates used in the open circuit configuration are extremelylow. CO₂ analyzer footprint, weight, and power requirements arepreferably 0.014-0.028 m³ (0.5-1.0 ft³), 6.8-9 kg (15-20 lbs), and 10-20W, respectively.

Having illustrated and described the principles of my invention in apreferred embodiment thereof, it should be readily apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. I claim all modificationscoming within the spirit and scope of the accompanying claims.

We claim:
 1. A process for determining the concentration of CO₂ orp(CO₂) a seawater sample, which comprises:providing a control solutionincluding an alkanolamine chemical reagent, that absorbs largequantities of CO₂ by a reversible reaction with CO₂ producing conductiveionic CO₂ reaction products; providing a membrane contactor having amembrane located therewithin for separating said seawater sample andsaid control solution, said membrane being permeable to CO₂ butimpermeable to conductive ionic CO₂ reaction products; measuring thespecific conductance of the control solution before exposure to CO₂ ;introducing said seawater sample and said control solution into saidmembrane contactor, the seawater sample flowing through the membranecontactor on the opposite side of the membrane from the controlsolution: transporting CO₂ from said seawater sample through themembrane into said control solution; chemically amplifying conductivespecies in the said control solution through reversible reactionsbetween CO₂ and the chemical reagent which produces said conductiveionic CO₂ reaction products; measuring the specific conductance of thecontrol solution including said conductive ionic CO₂ reaction products:and determining p(CO₂) or CO₂ concentration in the seawater sample bycomparing the difference between the specific conductance of the controlsolution including said conductive ionic CO₂ reaction products and thespecific conductance of the control solution before exposure to CO₂, andcomparing this difference to known p(CO₂) or CO₂ calibration values. 2.The process of claim 1, wherein the membrane is a microporous membraneor a non-porous membrane.
 3. The process of claim 1, wherein theseawater sample and the control solution flow counter-currently to eachother within the membrane contactor, thereby maximizing the CO₂concentration difference along the membrane contactor, and the passageof CO₂ into the control solution.
 4. The process of claim 1, wherein themembrane materials for the non-porous membranes comprisepolytetrafluoroethylene, polydimethylsiloxane, and the membrane materialfor the microporous materials is microporous polypropylene.
 5. Theprocess of claim 1, wherein the membrane is a microporous membranecomprising hollow fibers.
 6. The process of claim 5, wherein the hollowfibers comprise a coaxial tube within a tube arrangement for bothgas-liquid and liquid-liquid CO₂ exchange.
 7. The process of claim 1,wherein the specific conductance is measured by a flow-throughinstrument.
 8. The process of claim 1, wherein the process iscontinuous.
 9. The process of claim 1, wherein the specific conductanceof the control solution is detected using an conductivity measuringinstrument.
 10. The process of claim 2, wherein the seawater contains upto 30 parts per thousand of NaCl by weight.
 11. The process of claim 1,wherein the alkanolamine is selected from a group consisting of primary,secondary, and tertiary alkanolamines.
 12. The process of claim 1,wherein the alkanolamine is selected from a group consisting ofdiethanolamine, monoethanolamine, triethanolamine,N,N-dimethylethanolamine, N-methylethanolamine, N-ethylethanolamine, anddiisopropanolamine.
 13. The process of claim 1, wherein theconcentration of the conductive ionic CO₂ reaction products areincreased by at least about a factor of 25 fold as compared to CO₂reaction products formed without amplification.
 14. The process of claim1, which comprises a closed circuit configuration in which the controlsolution continuously recirculates through the membrane contactorreversibly gaining or losing CO₂ from the seawater sample, depending onthe CO₂ concentration gradient across the membrane, giving rise tospecific conductance in the control solution as the chemical reagentreversibly reacts with CO₂ which are proportional to the p(CO₂) of thesample phase.
 15. The process of claim 1, which comprises an opencircuit configuration in which chemical reagent solution flows from afeed reservoir, first through the membrane, then measuring the specificconductance of the control solution including said conductive ionic CO₂reaction products and then to a waste repository, the chemical reagentbeing expendable and making a single pass through the system.
 16. Theprocess of claim 1, which comprises a stopped flow injectionconfiguration in which the chemical reagent is injected into themembrane contactor, the flow is stopped, and after a predeterminedcontact time, flow is re-established, a plug of CO₂ -containing chemicalreagent solution being displaced from the membrane, and then measuringthe specific conductance of the control solution including saidconductive ionic CO₂ reaction products.
 17. A process for continuouslydetermining the concentration of CO₂ or p(CO₂) in a seawater sample,which comprises:providing a control solution, including an alkanolaminechemical reagent selected from a group consisting of primary, secondary,and tertiary alkanolamines, that absorbs large quantities of CO₂ by areversible reaction with CO₂ producing conductive ionic CO₂ reactionproducts; providing a membrane contactor having a membrane locatedtherewithin for separating said seawater sample and said controlsolution, said membrane being permeable to CO₂ but impermeable toconductive ionic CO₂ reaction products; continuously measuring thespecific conductance of the control solution before exposure to CO₂using a flow-through instrument; continuously introducing said seawatersample and said control solution into said membrane contactor, theseawater sample flowing through the membrane contactor on the oppositeside of the membrane from the control solution; continuouslytransporting CO₂ from said seawater sample through the membrane intosaid control solution; continuously chemically amplifying conductivespecies in the control solution through reversible reactions between CO₂and the chemical reagent which produce said conductive ionic CO₂reaction products, the conductive ionic CO₂ reaction products beingincreased by at least about a factor of 25 fold as compared to CO₂reaction products formed without amplification; continuously measuringthe specific conductance of the control solution including saidconductive ionic CO₂ reaction products using a flow-through instrument;and continuously determining p(CO₂) or CO₂ concentration in the seawatersample by comparing the difference between the specific conductance ofthe control solution including said conductive ionic CO₂ reactionproducts and the specific conductance of the control solution beforeexposure to CO₂, and comparing this difference to known p(CO₂) or CO₂calibration values.
 18. A process for continuously determining theconcentration of CO₂ or p(CO₂) in a seawater sample, whichcomprises:providing a control solution, including an alkanolaminechemical reagent, that absorbs large quantities of CO₂ by a reversiblereaction with CO₂ producing conductive ionic CO₂ reaction products;providing a membrane contactor having a membrane located therewithin forseparating said seawater sample and said control solution, said membranebeing permeable to CO₂ but impermeable to conductive ionic CO₂ reactionproducts; continuously measuring the specific conductance of the controlsolution before exposure to CO₂ using a flow-through instrument;continuously introducing said seawater sample and said control solutioninto said membrane contactor, the seawater sample flowing through themembrane contactor on the opposite side of the membrane from the controlsolution; continuously transporting CO₂ from said seawater samplethrough the membrane into said control solution; continuously chemicallyamplifying conductive species in the control solution through reversiblereactions between CO₂ and the chemical reagent which produce saidconductive ionic CO₂ reaction products; continuously measuring thespecific conductance of the control solution including said conductiveionic CO₂ reaction products using a flow-through instrument; andcontinuously determining p(CO₂) or CO₂ concentration in the seawatersample by comparing the difference between the specific conductance ofthe control solution including said conductive ionic CO₂ reactionproducts and the specific conductance of the control solution beforeexposure to CO₂, and comparing this difference to known p(C₂) or CO₂calibration values.